The present invention relates to a battery comprising a case formed by a resin sheet.
In recent years, electronic apparatus such as portable wireless telephone, portable personal computer and portable video camera have been developed, and various electronic apparatus have been reduced in size to an extent such that they are portable. This requires the use of batteries having a high energy density and a light weight as batteries to be incorporated in these electronic apparatus. A typical battery that satisfies these requirements is a nonaqueous secondary electrolytic battery comprising as a negative electrode material a lithium intercalation compound having lithium metal, lithium alloy or lithium ion occluded in a carbon-based material as a host material (The term xe2x80x9chost materialxe2x80x9d as used herein is meant to indicate a material capable of occluding and releasing lithium ion), as a positive electrode material a compound which undergoes reversible electrochemical reaction with lithium ion such as lithium-cobalt composite oxide and as an electrolytic solution an aprotic organic solvent having a lithium salt such as LiClO4 and LiPF6 dissolved therein.
This nonaqueous secondary electrolytic battery comprises a negative electrode plate having a negative electrode compound containing the foregoing negative electrode material retained on a negative electrode collector as a support, a positive electrode plate having a positive electrode compound containing the foregoing positive electrode material retained on a positive electrode collector as a support and a separating material provided interposed between the negative electrode plate and the positive electrode plate. The separating material often means a separator for retaining the electrolytic solution as well as interposing between the negative electrode plate and the positive electrode plate to prevent the shortcircuiting of the two electrodes. However, a solid electrolyte which is disposed between the negative electrode plate and the positive electrode plate to prevent shortcircuiting as well as cause ionic conduction, too, is called a separating material.
The foregoing positive electrode plate and negative electrode plate, which are each in the form of thin sheet, are laminated on each other or spirally wound normally with a separating material interposed therebetween to form an electricity-generating element. The electricity-generating element thus is received in a battery case made of a metal such as stainless steel, nickel-plated iron or aluminum into which an electrolytic solution is then injected. The battery case is then hermetically sealed with a cover plate to assemble a battery.
The use of such a metallic battery case provides a high airtightness and an excellent mechanical strength but puts great restrictions on the reduction of the weight of the battery and the selection of the shape of the battery.
As an approach for solving the foregoing problem there has been proposed a structure comprising an electricity-generating element received in a battery case made of a resin sheet. This structure is advantageous in that it can reduce the weight of the battery and enhance the degree of selection of the shape of the battery. On the other hand, this structure has the following disadvantages and problems awaiting solution.
(a) A structure comprising as the foregoing resin sheet a resin sheet made of a laminate of a metal foil and a heat-fusible resin layer is advantageous particularly in that it gives a high airtightness. However, when the resin sheet is welded under tension, the metal foil undergoes cracking, deteriorating the sealing properties of the resin sheet. Thus, water enters into the battery, reducing the life of the battery.
(b) The structure having a battery case formed by this kind of a resin sheet is liable to peeling of the welded portion of the ends of the sheet which are superimposed on each other with the rise in the inner pressure of the battery. The metal-resin sheet exhibits a sufficiently enhanced airtightness in the direction perpendicular to the metal foil. However, the exterior and the interior of the battery case are separated not by the metal foil but by the resin layer sandwiched by two sheets of metal foil at the welded portion of the ends of the sheet which are superimposed on each other. Thus, water content and electrolytic components can easily pass though the resin layer.
(c) The production of this kind of a battery is often carried out by a process which comprises putting an electricity-generating element provided with lead terminals into an open battery case made of a resin sheet, injecting an electrolytic solution into the battery case, and then welding the opening portion of the battery case with the lead terminals put between the two sides of the opening portion to seal the battery case. However, since the electrolytic solution hits and bounces off the electricity-generating element or is attached to the lead terminals during injection into the battery case, welding is occasionally carried out with the electrolytic solution attached to the area of the resin sheet to be welded. This also causes the deterioration of the sealing properties.
(d) The spirally-wound electricity-generating element, if used, can be easily loosed when received in a battery case made of a resin sheet unlike in the metallic case. Eventually, the gap between the various electrode plates cannot be kept constant over an extended period of time. Accordingly, this structure tends to show an decrease of discharge capacity after repeated charge-discharge cycles.
(e) At the first step of forming a battery case from a resin sheet, two parallel sides of the resin sheet are welded to each other to form a cylinder. However, the welded portion protrudes from the battery case when the battery is assembled. Thus, this protrusion interferes in the accumulation of a plurality of the batteries. Accordingly, the accumulation of these batteries can easily produce an unnecessary space or takes time to compress the stack of these batteries so that the protrusion doesn""t interfere.
(f) In this structure, the electricity-generating element is subject to less pressure than the conventional case comprising a metallic case. Accordingly, in the case where the battery comprising a case made of a resin sheet is used in a vibrational atmosphere, the electricity-generating element can easily move in the battery case, occasionally breaking the lead terminals at the point between the fixed area at the heat-welded portion and the electricity-generating element. Further, since the electricity-generating element is subject to small pressure, there occurs nonuniformity in the distance between electrodes in the electricity-generating element, causing the deterioration of charge-discharge properties.
The present invention has been worked out in the light of the foregoing circumstances. An object of the present invention is to provide a battery having a long life which exhibits a sufficiently enhanced airtightness while attaining the reduction of the weight of the battery case made of a resin sheet. Another object of the present invention is to prevent the welded portion of the two ends of resin sheet from interfering in the accumulation of batteries, enhancing the volume energy density of the entire battery. A still another object of the present invention is to prevent the breakage of lead terminals in the battery case.
The battery according to the invention comprises the following elements: an electricity-generating element comprising a positive electrode plate and a negative electrode plate which are disposed opposed to each other with a separating material interposed therebetween and lead terminals connected to the respective electrode plates and a battery case made of a resin sheet (formed by laminating an oriented resin layer on both surfaces of a metal layer) which is open in one or two directions before sealing and then welded at its openings with the foregoing electricity-generating element received therein and the foregoing lead terminals inserted through one of the openings to seal the case.
As the metal layer constituting the resin sheet there may be used an aluminum foil, aluminum alloy foil, titanium foil or the like. The number of the resin layers and metal layers constituting the resin sheet is not limited to one. Two or more resin layers and metal layers may be used to form the resin sheet. The resin layer may be in the form of a multi-layer structure comprising layers of the same or different materials. The structure of the resin sheet is not limited to resin layer-metal layer-resin layer structure but may be resin layer-resin layer-metal layer-resin layer structure or resin layer-metal layer-resin layer-metal layer-resin layer structure. In short, the metal layer is little liable to cracking to provide improvements in sealing properties so far as a resin layer which has been oriented is provided on both surfaces of the metal layer. The term xe2x80x9cprovided on both surfaces of the metal layerxe2x80x9d as used herein does not necessarily mean that the resin layer comes in contact with the metal layer. For example, the resin sheet may have an oriented resin layer-unoriented resin layer-metal layer-unoriented resin layer-oriented resin layer structure. Thus, the structure having a thin unoriented resin layer is substantially included in the present invention.
In order to weld the opening of the resin sheet, it is necessary that a thermoplastic high molecular material such as polyethylene, polypropylene and polyethylene terephthalate be present on the surface layer of the resin sheet.
In a preferred embodiment of the electricity-generating element to be used in the invention, a positive electrode plate and a negative electrode plate are wound with a separating material interposed therebetween to make an ellipsoidal coil having an ellipsoidal section almost perpendicular to the axis thereof. However, the present invention is not limited to this shape. The electricity-generating element according to the invention may be in any form such as coil having a circular or noncircular section, stack of flat electrode plates with a separating material interposed therebetween and stack of sheet-shaped electrodes folded with a separating material interposed therebetween. The term xe2x80x9cellipsoidxe2x80x9d as used herein is meant to indicate a shape obtained by combining a nearly semi-arch portion and a nearly linear portion in such an arrangement that a pair of opposing semi-arc portions are connected to each other at the ends thereof with two parallel linear portions.
In the case where an ellipsoidally wound electricity-generating element is received in the battery case made of a resin sheet which is open in one or two directions, the winding axis of the ellipsoidally wound electricity-generating element is preferably perpendicular to the plane of the opening of the unsealed battery case. The term xe2x80x9cperpendicularxe2x80x9d as used herein means not only xe2x80x9ccompletely perpendicularxe2x80x9d but also xe2x80x9calmost perpendicularxe2x80x9d.
The electricity-generating element according to the invention comprises a positive electrode, a negative electrode and a separating material in combination. The positive electrode plate is prepared by applying a positive electrode compound obtained by mixing an active positive electrode material, a binder and an electrically-conducting agent described later to one or both surfaces of an electrically-conductive sheet-shaped or foil-shaped collector, and then drying the coated material. The negative electrode plate is prepared by applying a negative electrode compound obtained by mixing an active negative electrode material and a binder described later to one or both surfaces of an electrically-conductive sheet-shaped or foil-shaped collector, and then drying the coated material.
In the case where the separating material prevents the shortcircuiting of the two electrodes, it is usual that the electrolytic solution is injected into the battery case prior to sealing of the battery case. However, the electricity-generating element may comprise in combination a positive electrode, a negative electrode and a solid electrolyte which also acts as a separating material. The foregoing solid electrolyte may be an organic material, an inorganic material or a combination thereof or may be a high molecular electrolyte or porous material impregnated with an electrolytic solution.
As a nonaqueous electrolytic solution to be used as an electrolyte there may be used a known material. Examples of such a nonaqueous electrolytic solution include polar solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, xcex3-butyrolactone, sulfolane, dimethylsulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane and methyl acetate, and mixture thereof.
Examples of lithium salt to be dissolved in the organic solvent include salts such as LiPF6, LiClO4, LiBF4, LiAsF6, LiCF3CO2, LiCF3SO3, LiN(SO2CF3), LiN(SO2CF2CF3), LiN(COCF3) and LiN(COCF2CF3)2, and mixture thereof.
Further, an electrolyte layer mainly composed of an organic material, an inorganic material or a mixture thereof may be formed on the upper surface of the positive electrode compound layer and/or negative electrode compound layer. The electrolyte layer is essentially required to be chemically or electrochemically stable in the battery and have a raised mechanical strength. The electrolyte layer is preferably made of a solid electrolyte. However, the electrolyte layer doesn""t need to be made of a single component as a whole. Further, the electrolyte layer doesn""t need to be made of an electrolyte as a whole. For example, a solid electrolyte which has been impregnated with an electrolytic solution to have an enhanced conductivity or a known separating material impregnated with an electrolyte may be used. In other words, lithium ion can be smoothly transferred at the interface of the electrolyte layer with the electrode plates or in the electrolyte plate through the electrolytic solution. However, if an organic electrolytic solution is contained in the electrolyte layer, the charging voltage cannot be raised beyond the decomposition voltage of the organic electrolytic solution, restricting the range of selection of the active material to be used. Accordingly, an electrolyte layer free of organic electrolytic solution is preferably used to enhance the degree of freedom of selection of materials.
When an organic solid material such as polyethylene oxide, polyacrylonitrile, polyethylene glycol and modification product thereof is used as a constituent of the solid electrolyte, little cracking occurs during winding because such an organic solid material is lighter and more flexible than inorganic solid materials. On the other hand, when the constituent of the solid electrolyte is a lithium-ionically conductive inorganic solid material such as lithium-lanthanum perovskite and lithium-ioncially conductive glass, the resulting solid electrolyte exhibits a high heat resistance and hence an excellent reliability at high temperatures.
In addition, when the constituent of the electrolyte layer is a mixture of an organic material and an inorganic material, the resulting electrolyte layer allows the two components to compensate for defect of the other while accomplishing the advantage of the two components. In other words, even if the organic material in the mixture melts, it can be retained by the inorganic material and thus doesn""t flow away. Even when the inorganic material is contained in a large amount, the organic material acts as a binder, making it possible to prevent cracking. In the case where the constituent of the electrolyte layer is a mixture, if one of the components of the mixture is an electrolyte, the other may be a nonelectrolyte which is an inorganic material (inorganic filler) such as magnesium oxide, silicon oxide and calcium salt of silicon oxide or a mixture thereof. The mixture may comprise an inorganic material in an amount of from 70 to 85%, an organic solid material in an amount of from 10 to 15%, and the balance of other components (e.g., binder such as polyvinylidene fluoride). An electrolyte salt may or may not be used depending on the constitution.
As the separating material constituting the electricity-generating element according to the invention there may be used an insulating polyethylene microporous membrane impregnated with an electrolytic solution, high molecular electrolyte or gel electrolyte comprising a high molecular electrolyte impregnated with an electrolytic solution. Alternatively, an insulating microporous membrane, a high molecular solid electrolyte, etc. may be used in combination. When a porous high molecular solid electrolyte layer is used as a high molecular solid electrolyte, the electrolyte with which the high molecular material is impregnated may be different from the electrolyte to be incorporated in the pores.
The active positive electrode material is not specifically limited. Examples of an inorganic compound to be used as such an active positive electrode material include composite oxide represented by the composition formula LixMO2 or LiyM2O4 (wherein M represents a transition metal, x represents a number of from 0 to 1, both inclusive, and y represents a number of from 0 to 2, both inclusive), oxide having tunnel-like pores, and laminar metal chalcogen compound. Specific examples of these inorganic compounds include LiCoO2, LiNiO2, LiMn2O4, Li2Mn2O4, MnO2, FeO2, V2O5, V6O13, TiO2, and TiS2. Examples of an organic compound to be used as an active positive electrode material include electrically-conductive polymers such as polyaniline. The foregoing various active materials may be used in admixture regardless of which they are inorganic or organic.
The active negative electrode material is not specifically limited. Examples of the active negative electrode material employable herein include oxides of Al, Si, Pb, Sn, Zn and Cd, alloy of these metals with lithium, transition metal oxides such as LiFe2O3, WO2 and MoO2, carbon-based materials such as graphite and carbon, lithium nitrides such as Li5(Li3N), metal lithium foil, and mixture thereof.
In the case where the two ends of the opening of the resin sheet are welded to each other to seal the battery case, the sealing properties of the battery case can be further enhanced by melting and solidifying the resin to form a mass at the welded portion more inside the battery case than the inner end of the welded portion.
The battery case made of a resin sheet employable herein may be in any form such as cylindrical laminate case formed by heat-fusing a metal-laminated resin film, case obtained by heat-fusing four sides of two metal-laminated resin sheets, case obtained by double-folding a resin sheet and heat-fusing three sides of the folded sheet, and cup-shaped laminate case obtained by press-molding a metal-laminated resin sheet such that the electricity-generating element can be received.
In order to enhance the sealing properties of the battery case, the thickness of the welded portion at which two edges of the opening of the resin sheet are welded to each other to seal the battery case may be predetermined smaller at the outer end of the battery case than at the inner end of the battery case. It is thought that the reduction of the thickness of the outer end reduces the contact area with the atmosphere, enhancing the sealing properties.
In the case where a battery case made of a resin sheet which has been sealed with the foregoing electricity-generating element received in the interior thereof is prepared, it is preferred that two parallel sides of a rectangular resin sheet are opposed and welded to each other to form a cylinder which is open in two directions (this welded portion will be hereinafter referred to as xe2x80x9cwelded portion Xxe2x80x9d). With the lead terminals of the foregoing electricity-generating element put in one of the two openings, the two edges of the openings are then opposed and welded to each other to form a sealed bag, thereby forming a structure having the surface of the welded portion X fixed to the surface of the battery case. By fixing the welded portion X to the surface of the battery case, the welded portion X can be prevented from protruding from the battery case. Thus, no unnecessary space can be produced when a plurality of batteries are stacked. Further, this structure facilitates stacking of batteries.
Similarly, in the case where a bag-shaped battery case made of a resin sheet which has been sealed with an electricity-generating element received in the interior thereof is prepared, the battery case is prepared by welding two parallel sides of a rectangular resin sheet opposed to each other to form a cylinder which is open in two directions (this welded portion will be hereinafter referred to as xe2x80x9cwelded portion Xxe2x80x9d). With the lead terminals of the foregoing electricity-generating element put in one of the two openings, the two edges of the openings are then opposed and welded to each other to form a sealed bag (the portion welded with the lead terminals put therein will be hereinafter referred to as xe2x80x9cwelded portion Yxe2x80x9d, and the other welded portion will be hereinafter referred to as xe2x80x9cwelded portion Zxe2x80x9d), thereby forming a structure having the surface of the welded portion X fixed to the surface of the welded portion Y and/or welded portion Z. In this arrangement, too, the welded portion X can be prevented from protruding from the battery case. Thus, no unnecessary space can be produced when a plurality of batteries are stacked. Further, this structure facilitates stacking of batteries.
In order to fix the welded portion X to the surface of the battery case or the welded portion Y and/or welded portion Z, a proper adhesive agent may be used depending on the material of the resin sheet. Alternatively, they may be heat-fused to each other.
In the case where a fixing tape is wound on the coiled electricity-generating element along its winding axis, the welded portion X of the battery case is preferably positioned so as to partly overlaps the fixing tape. The coiled electricity-generating element is always pressed at the portion where the welded portion X and the fixing tape overlap each other, keeping the distance between the electrodes constant during charge-discharge cycle and hence minimizing the decrease of capacity with the increase of the number of cycles.
In the case of battery provided with bent lead terminals, the lead terminals are preferably curved at a radius of from 0.5 mm to 4 mm. When the radius of curvature falls below 0.5 mm, the lead terminals can easily break. On the contrary, when the radius of curvature exceeds 4 mm, the space efficiency is lowered.
In the structure comprising an electricity-generating element received in a closed battery case made of a resin sheet, it is preferably arranged that the electricity-generating element has lead terminals which linearly extend to the exterior of the battery case and the inner pressure in the sealed battery case is lower than ordinary atmospheric pressure, i.e., 760 mmHg. This is because the difference between the inner pressure and ordinary atmospheric pressure causes the electricity-generating element to be clamped, keeping the distance between the electrodes constant during charge-discharge cycle and hence minimizing the decrease of capacity with the increase of the number of cycles.
In order to inject the electrolytic solution into the battery case made of a resin sheet, it is preferred that a foam-preventive material made of an insulating net, unwoven cloth, felt or porous material be disposed along the winding axis of the electricity-generating element in contact with the electricity-generating element to receive the electricity-generating element in the battery case, followed by the injection of the electrolytic solution from the foam-preventive material side. In some detail, an electricity-generating element having an insulating foam-preventive material fixed thereto is received in a battery case which is open in two directions. The battery case is then welded and sealed at the opening where the foam-preventive material is not disposed. Thereafter, an electrolytic solution is injected into the battery case through the other opening where the foam-preventive material is disposed. Thereafter, the battery case is evacuated. The two opposing edges of the other opening are then welded to each other to seal the battery case. Since the electrolytic solution moves into the electricity-generating element while permeating into the foam-preventive material during the injection, the electrolytic solution can be prevented from hitting and bouncing off the electricity-generating element and hence can be prevented from being attached to the inner area of the battery case close to the opening. Further, since the upper end of the electricity-generating element is covered by the foam-preventive material, even when bubbles burst in the electrolytic solution during evacuation, no spray can be scattered out of the electricity-generating element, making it possible to prevent the electrolytic solution from being attached to the sealed portion and hence prevent the deterioration of the sealing properties.
Examples of the foregoing foam-preventive material include net, unwoven cloth, felt and porous material made of a polyolefin such as polypropylene and polyethylene. In order to fix the foam-preventive material to the electricity-generating element, a means such as heat fusion and bonding can be employed. As the adhesive there is preferably used an adhesive material which has heretofore been used in adhesive tapes. Examples of such an adhesive material include silicon-based adhesive, rubber-based adhesive, and acrylic adhesive.